Resistor grid for electric furnaces



F. T. COPE 2,004,851

RESISTOR GRID FOR ELECTRIC FURNACES Filed March 1, 1932 4 Sheets-Sheet 1 June 11, 1935.

June 1l, 1935. F. T. COPE n A RESISTOR GRID FOR ELECTRIC FURNACES 4 Sheets-Sheet 2 Filed March l, 1932 June 1l, 1935. F. T. COPE 2,004,851

RESISTOR GRID FOR ELECTRIC FURNACES Filed March l, 1932 4 SheeS-Sheet 5 Pfalz/f7. Hope June 11, 1935. F, T, COPE 2,004,851

RESISTOR GRID FOR ELECTRIC FURNACES Filed March 1, 1932 4 Sheets-Sheet 4 Ffa/lli 7.' (blue.

, @gem Q5/M Patented `une 11, 1935 UNITED STATES PATENT OFFICE Frank T. Cope, Salem, Ohio, assignor to The Electric Furnace Company, Salem, Ohio, a corporation of Ohio Application March 1, 1932, Serial No. 596,034

16 Claims.

The invention relates to resistance elements for electric furnaces and the like and more particularly to a cast resistor grid.

The resistor grids to which the invention pertains are especially adapted for the heating of large industrial furnaces such as are used for annealing, heat treating, normalizing, etc. and a number of considerations are involved in the design of such resistor grids.

Beside being mounted on the side walls of the furnaces, these grids are also frequently mounted just below the roof or spaced above the floor of the furnace, in which case the grids are supported at their ends as a beam.

Furthermore, it is desirable to have all of the grids, wherever mounted, positioned so that the apex of the grid in cross section is directed toward the interior of the furnace. It is therefore necessary that the grids, when mounted as a beam with the apex directed toward the in terior of the furnace, have a sufficient strength for supporting their own weight satisfactorily. It will be seen that by mounting the grids with the apex toward the interior of the furnace, there will be obtained a maximum surface radiation from the resistor grid toward the interior of the furnace per unit length of the grid.

Such resistor grids are preferably formed of a heat resisting alloy whose properties render it suitable for the service intended. One alloy which is suitable for many uses contains nickel,

chromium and iron. For certain special uses it may be necessary to use alloys containing only iron and chromium or only nickel and chromium. It is a characteristic of all such alloys that the shrinkage upon solidification is high and that, therefore, shrinkage cavities tend to form wherever there is a region in the casting which solidiiies materially later than other regions. For this reason experience shows that sound castings can be had only if the cross sectional shape is such as to present substantially uniform thickness at all points, preferably without any projecting ribs.

It may be seen that uniform cooling depends upon two things, the first of which is the uniformity of section and the second of which is the provision of an adequate amount of sand to absorb the heat from the hot metal. For instance, in a casting of V-shaped section, as will be referred to hereinafter, the actual thickness at the apex of the V may be the same as at other points but the small point of sand which forms the interior surface at the apex has such little cooling effect that the last solidication will occur at this point and the casting will be unsound.

For the purpose of making sound castings it would seem that a relatively thin cross section would be ideal and this is indeed a fact. However, such a shape would not be suitable for use in furnaces because the grid thus formed would have adequate strength in one direction but quite insufficient strength in the other. Furthermore, the heat radiating properties of such a section would be inferior to that which forms the subject of the present invention.

I have found that a grid of the form described herein presents the best combination of desirable qualities which have so far been attained. It is so formed as to radiate the major portion of its heat toward the interior of the furnace. It may be cast of heat resisting alloy with the assurance of sound castings substantially free from shrinkage cavities and other internal defects. It has a cross section such that the desirable rigidity may be provided whereby such grids may readily be mounted in a horizontal or nearly horizontal position without failure from sagging. It is of a shape which will shed any scale or dirt which falls upon it when it 2 is mounted with the apex upward in the floor or bottom of a furnace.

In the development of these cast resistor grids, applicant and his associates have, during the past ten years, designed a number of different cross sectional shapes of resistor grids, all of which have been used more or less extensively in large vindustrial furnaces.

The earliest of these designs is a substantially T-shaped cross section as shown in Cope Patent No. 1,767,172. Experience has shown that this T section based upon the usual cross sectional area has comparatively little beam strength and furthermore, that it is very diflicult to cast because of the tendency to develop segregation and shrinkage cavities at the junction of the stem with the crossbar of the T.

The channel shape as disclosed in the above mentioned patent and claimed in the division thereof, Cope Patent No. 1,767,171, has great physical strength in a direction parallel to the base of the channel but is comparatively weak in the opposite direction. It is also subject to the formation of shrinkage cavities at the junction of the base and fianges.

The V section with tapered wings as disclosed in Baily Patent No. 1,864,334, may be made so as to give more physical strength than the T section. However, the thickness is not uniform and furthermore, the greatest thickness is encountered at the junction of the two wings where, because of the comparatively sharp angle between the wings, the cooling from the sand of the mold is least effective. Therefore this grid tends to develop a shrinkage cavity in the region of this junction. Also because of this comparatively sharp angle there is a tendency for the sand of the mold to wash away before the flow of the metal, which not only causes lumps upon the casting but also results in sand inclusions elsewhere within the metal.

The last mentioned objection also applies to a lesser extent to the Y .section disclosed in Cope l and Vaughan Patent No. 1,851,161. This section is a fairly desirable one because of its good physical strength and presents a relatively easy molding problem.

The object of the present improvement is to p rovide a resistor grid of such cross sectional shape that the objections and disadvantages of the former grid sections are overcome; and to produce a resistor grid having high physical strength and having a cross section of uniform thickness and a .stream line arrangement of metal, thus permitting great ease in molding and giving uniform chilling effect to all surfaces of the casting and producing exceptionally sound structure in the finished casting.

Further objects of the improvement are to provide a grid section of such shape that there is no portion thereof which will cause a shrinkage cavity to form; andto provide a smoothly molded or rounded apex upon the section which not only gives uniform chilling effect but obviates sharp corners in the sand which might tend to wash away when pouring the metal.

A further object is to provide a grid section of such shape that it will easily and readily shed scale and dirt when placed in horizontal position in the floor of a furnace and in which the warping due to the high temperatures at which the grids are operated, is considerably minimized.

Another object is to provide a resistor grid of such cross sectional shape that when placed adjacent to a furnace wall with the apex of the grid toward the interior of the furnace, it will direct the majority of the heat toward the interior of the furnace While nearly all of the remaining portion of the heat will be directed toward the furnace wall at points on either side of the grid so as to heat the portions of the wall not covered by the grid whereby this additional heat from the furnace wall may be directed toward the interior of the furnace.

A still further object is to provide the improved grid with integral tail portions for supporting the grid; these tail portions being provided at opposite ends of those grids which are to be supported in horizontal or nearly horizontal position in the furnace roof or floor and the tail portions being so positioned that whether the grid is carried with the apex up or down, the lower surface of the grid and the tails will lie substantially in the same plane whereby the grid is free to expand and contract without meeting interference, as would be the case if the bearing surface of the tails were on a different plane from the lower surface of the main portion of the grid.

The above and other objects may be accomplished by providing a resistor grid having a cross sectional shape which might be termed a V-shape with a rounded apex both the inside and outside thereof being rounded upon substantially the same center, or a U-shape with outwardly flared flat legs; the grid being preferably formed as shown in the various prior patents and applications above referred to, comprising a plurality of straight bar portions connected at alternate ends by bends; the cross section of the grid being of uniform thickness and the cross sectional area of the bends being slightly greater than the cross sectional area of the straight bar portions since the liberation of heat from the bends is to a slight extent interfered with by the supporting means, and because, due to the sharpness of the bend, the electric current tends to concentrate around the shortest path which is at the inner circumference of the bend.

An embodiment of the invention thus set forth in general terms is illustrated in the accompanying drawings, in which l Figure 1 is a sectional View through a large industrial furnace showing the improved resistor grids mounted upon the side walls, in the roof and in the floor of the furnace;

Fig. 2, a front elevation, upon a larger scale, of one of the side wall resistor grids showing the manner of mounting the same upon the side wall of the furnace;

Fig. 3, a plan section, upon a still larger scale, of a portion of the side wall of the furnace and a portion of the side wall grid which is mounted thereon, as taken on the line 3 3, Fig. 1;

Fig. 4, a fragmentary longitudinal sectional view through a portion of the furnace roof showing a portion of one of the grids mounted in the roof in section as taken on the line I Il, Fig. 1;

Fig. 5, a similar section through the floor of the furnace showing a portion of one of the floor grids in section taken as on the line 5, Fig. 1;

Fig. 6, a transverse section on an enlarged scale through one of the straight bar portions of the grid taken on the line 6 6, Fig. 2;

Fig. '7, a similar section through one of the bends of the resistor grid taken as on the line 1 1, Fig. 2;

Fig. 8, a cross section through one of the bends of the roof grid showing the tail portion in the same plane with the apex of the grid section; and

Fig. 9, a similar section through one of the bends of a floor grid showing the tail portion in the same plane with the base of the grid section.

Similar numerals refer to similar parts throughout the drawings.

Referring particularly to Fig. 1 of the drawings, an industrial furnace of suitable design is more or less conventionally illustrated and 4comprises the refractory floor III and side walls II which may be supported upon the structural members I2, an arched roof I3, also of refractory material, enclosing the working chamber indicated generally at I4.

The furnace structure may be enclosed within a steel shell I 5 as in usual practice and supported as by the buckstays I6 connected at their upper ends by tiebars Il.

The improved resistor grid to which the invention pertains is of a cross sectional shape, such as indicated generally at I8, being what may be termed a V-shape with a rounded apex I9 both the inside and outside thereof being rounded upon substantially the same center or this cross sectional shape might be described as U-shape with the flat legs of the U diverging or spread apart, as indicated at 20. The apex I9 and legs 20 are of uniform thickness and the extreme edges of the legs are preferably beveled as at 20.

'I'his does not nullify the effect of uniform thickness, and is desirable in order to reduce the likelihood of a flash` or fin upon the casting.

As shown in the cross sectional views of the drawings, any-given cross section of the grid is of uniform thickness throughout, and it will also be seen that a proper stream line arrangement of the metal is produced by this cross sectional shape, thus producing a grid section which may be easily and properly cast from heat resisting alloy and which provides for uniform cooling of the cast resistor grid and consequently freedom from shrinkage cavities. Also since there are no sharp corners in the sand mold, due to the rounded apex of the V-shape or U-shape grid illustrated, there is no tendency for sand to wash away in the pouring of the metal.

The resistor grid may comprise a plurality of straight bar portions indicated at 2| connected at opposite ends by the bends 22 which may be substantially semicircular as best shown in Fig. 2, producing the more or less sinuous shape shown in said figure. The terminal ends 23 of the resistor may be connected to electric terminals in any usual and well known manner.

The vertically disposed resistors indicated at A are supported from the side walls I I of the furnace, being preferably spaced from said walls as shown on the drawings. For this purpose hooks 24, imbedded in the side walls Il, may engage the upper bends or loops of the grid so as to suspend the grid therefrom adjacent to the furnace wall.

Depending tail pieces 25 may be integrally formed upon the lower bends of the resistor and are extended through suitable openings in the guides 26 carried by the side walls Il. The straight bar portions 2| of the grid are thus permitted to expand and contract due to temperature changes within the furnace.

The roof grids indicated generally at B are adapted to be supported adjacent to the roof I3 of the furnace preferably with the apex of each grid directed downward toward the interior of the furnace, as best shown in Fig. 4.

Tail pieces 25amay be integrally formed at the bends at opposite ends of the roof grid, these tail pieces preferably adjoining the apex of the grid and resting upon ledges 21 located adjacent to the roof for the purpose of supporting the roof grids.

The lower surfaces of the grid and the tail pieces thus lie in the same plane so that each grid is free to expand and contract without meeting interference as would be the case if the bearing surfaces of the tail pieces were on a higher plane than the lower surface of the main portion of the grid.

The floor grids, indicated generally at C, as best shown in Fig. 5, are located with the apex upward or directed toward the interior of the furnace not only for the purpose of directing the major portion of the heat toward the interior of the furnace but also in order to permit the grids to shed any scale and dirt which may fall from the work.

These floor grids are preferably supported upon ledges 28 spaced above the floor so as to permit scale and dirt to collect upon the floor without contacting with the grids and causing shortcircuit in the same.

In order to support these floor grids upon the ledges 28, tail pieces 25b are formed integrally with the bends of said grids adjoining the base portion of the same so that the lower surfaces of the grid and the tail pieces lie in the same plane, permitting the grid to freely expand and contract without interference.

As best shown in Figs. 7, 8, and 9, the bends 22 in the grids have slightly greater cross sectional areas than the normal gridV section for the purpose of. compensating for the interference of the supporting means with the liberation of heat from the bends and also compensating for the tendency of the current to concentrate around the inner circumference of the bend.

The improved grid section above described has high physical strength, permitting it to be supported as a beam in substantially horizontal position for use in the roofs and floors of furnaces.

The stream line effect, absence of sharp reentrant angles, and uniform thickness of the cross section make it especially' adapted for casting from heat resisting alloy, producing structurally sound castings Without shrinkage cavities and substantially free from other internal defects such as sand inclusions.

It is well known that even heat resisting alloy metals are subject to what is termed "permanent growth; that is, under the usual conditions of service they increase in size. Under some conditions this also results in the pieces losing their original form and becoming seriously warped. While there are other contributing causes for this growth, the chief cause is that certain portions of the metal are heated to a higher temperature than other portions and the resulting non-uniform expansion causes minute deformation of the piece. This when repeated over long periods of time usually resulted in elongation because the stresses due to non-uniform temperature can more easily produce elongation than compression.

I have found that with the resistor grids heretofore used such as those of the T-shape and channel shape, this permanent growth commonly resulted in increasing the radius of curvature at the bends and thus in time causing adjacent loops finally to touch one another. It is of course obvious that with the method of support used it is not possible for the spacing of the loops to change, for which reason the foregoing result 1s produced.

It has been found by actual use that a grid of the present form under consideration shows such permanent growth at a very much slower rate than any other form of grid with which I am familiar.

I claim:

1. A cast metal resistor grid including a straight bar of substantially V cross sectional shape with an apex rounded both on the inside and outside on substantially the same center.

2. A cast metal resistor, grid including a straight bar of substantially V cross sectional shape with an apex rounded on the inside and outside, and of uniform thickness.

3. A cast metal resistor grid including a straight bar of substantially V cross sectional shape with an apex rounded both on the inside and outside, the apex and both ribs of the grid being of uniform thickness.

4. A cast metal resistor grid formed of a bar of substantially V cross sectional shape throughout with an apex rounded both on the inside and outside on substantially the same center, the bar being curved at intervals to provide a plurality of parallel portions connected at alternate ends by curved portions.

5. A cast metal resistor grid including a plurality of straight bars of substantially V cross sectional shape with an apex rounded both on the inside and outside on substantially the same center, and connected at their ends to produce a sinuous grid.

6. A sinuous cast metal resistor grid having a substantially V cross sectional shape with an apex rounded both on the inside and outside on substantially the same center.

7. A cast metal resistor of substantially V- shaped cross section, having sides of substantially uniform thickness and an apex rounded both on the inside and outside, and of the same thickness as the sides. y

8. A cast metal resistor grid formed of a bar of substantially V cross sectional shape throughout with an apex rounded both on the inside and outside on substantially the same center, the bar being curved at intervals to provide a plurality of parallel portions connected at alternate ends by curved portions, any given cross section being of substantially uniform thickness throughout.

9. A cast metal resistor grid including a straight bar of substantially U cross sectional shape with outwardly flared flat legs and an apex rounded both on the inside and outside on substantially the same center.

10. A cast metal resistor grid including a straight bar of substantially U cross sectional shape with a rounded apex and outwardly flared at legs all of uniform thickness.

11. A cast metal resistor grid formed of a bar of substantially U cross sectional shape throughout with outwardly ilared flat legs and a rounded apex, the bar being curved at intervals to provide a plurality of parallel portions connected qat alternate ends by curved portions.

12. A cast metal resistor grid including a piurality of straight bars of substantially U cross sectional shape throughout with outwardly flared ilat legs and an apex rounded both on the inside and outside on substantially the same center and connected at their ends to produce a sinuous grid.

13. A sinuous cast metal resistor grid having a substantially U cross sectional shape with outwardly ared at legs and an apex rounded both on the inside and outside on substantially the same center. Y

14. A cast metal resistor grid formed of a bar of substantially U cross sectional shape throughout with outwardly flared flat legs and an apex rounded both on' the inside and outside on subl.

resistor grid throughout and including the curved 2 portions being of substantially uniform V-shape with an apex rounded both on the inside and outside on substantially the same center.

16. A resistor grid comprising an integral castling of heat resisting material and including a 3 series of straight portions united at alternate ends by curved portions, the cross section of the resistor grid throughout and including the curved portions being of substantially uniform U-shape with outwardly ilared ilat legs and a rounded 3 apex.

FRANK T. COPE. 

